1. Field of the Invention
The present invention relates to an image heating apparatus suitable for being used as a heating fixing-device installed in a copier, a printer, etc., and in particular, to an image heating apparatus that uses a flexible sleeve.
2. Related Background Art
Heat roller type apparatuses are widely used as fixing apparatuses to heat and fix each unfixed toner image, which is indirectly or directly formed and born in a recording material, on a recording surface as a permanently fixed picture with proper image forming process measures such as electrophotographic recording technology and electrostatic recording technology. Recently, sleeve-heating type apparatuses have been put into practical use from the viewpoint of quick start and energy saving. In addition, electromagnetic induction heating type of fixing apparatuses heating each sleeve itself that consists of metal are also proposed.
a) Sleeve Heating Type of Fixing Apparatus
Sleeve heating type of fixing apparatuses are proposed in, for example, Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 2-157878, Japanese Patent Application Laid-Open No. 4-44075, Japanese Patent Application Laid-Open No. 4-204980, and the like.
That is, these are apparatuses of fixing each unfixed toner image on a surface of a recording material with each applied pressure of a nip section with giving the heat of a ceramic heater through a sleeve by forming the nip section with sandwiching the heat-resistant sleeve (fixing sleeve) between a ceramic heater serving as a heating body and a pressure roller serving as a pressure member, introducing a recording material, where the unfixed toner image is formed and born, between the sleeve of the nip section and the pressure roller, and sandwiching and conveying the recording material with the sleeve.
This sleeve heating type of fixing apparatus can constitute an on-demand type apparatus by using members with low thermal capacity for the ceramic heater and sleeve. Since what is necessary is to heat the ceramic heater heated to predetermined fixing temperature by energizing the ceramic heater, which is a heat source, only when image formation is executed, there are advantages to a short waiting time (quick start) from the power-on of a power supply of an image forming apparatus to a state ready for image formation, including a significantly small power consumption (power saving) at a standby state, etc.
b) Electromagnetic Induction Heating Type of Fixing Apparatus
In Japanese Utility Model Application Laid-Open No. 51-109739, an induction heating type of fixing apparatus is disclosed, the fixing apparatus which induces an eddy current in a metal layer of a fixing sleeve by a magnetic flux and makes the fixing sleeve heat by its Joule heat. This can make the fixing sleeve heated directly by using the generation of the induced current, and hence, achieves fixing process more highly effective than a heat roller type of fixing apparatus where a heat source is a halogen lamp.
Regardless of heating methods such as a sleeve heating method and an electromagnetic induction heating method, in color image fixing apparatuses, in order to improve image quality in special paper such as overhead transparency films (OHT films) and glossy paper, it is common for each apparatus to have a special print mode.
Since a highly glossy color image is often regarded as being in high quality generally, it is desirable in the print, which uses glossy paper, to fix a surface of the toner image more smoothly than in normal printing.
For this, it is effective to give plenty of heat in a fixing nip by slowly performing the fixing by slowing down fixing speed. A print mode that performs such fixing is called a gloss mode etc., and aims to obtain a glossy image by optimizing print speed, fixing temperature, etc.
In addition, in the fixing of an OHT film, the smoothness of a surface of a toner image is needed similarly to that in the gloss mode. Since OHT permeability decreases if sufficient fixing is not obtained, the image becomes a projection image that is impure or subfusc.
In the OHT mode also, it is possible, similarly to the gloss mode, to improve the OHT permeability by optimizing the fixing temperature by slowing down the fixing speed.
Nevertheless, there are the following problems when low-speed print like the gloss mode, OHT mode, etc. was performed in a fixing apparatus with the construction of using a flexible sleeve as a rotating body described above.
Since the inside surface of the sleeve and a supporting member frictionally contact with each other when the sleeve rotates, driving load is large. In order to decrease this driving load, it is very important to decrease the dynamical friction resistance between the inside surface of the sleeve and the supporting member. For example, as proposed in Japanese Patent Application Laid-Open No. 5-27619, slide property is secured by making lubricant such as heat-resistant grease intervene between the inside surface of the sleeve and the supporting member. In addition, the slide property is secured by decreasing a contact area of the sleeve with the supporting member by providing a rib in the sleeve supporting member.
Nevertheless, since it becomes difficult at the time of low-speed printing for grease to enter into an interface between a sliding plate and the sleeve because driving load rises due to high viscous drag of grease coated for securing slide property, and the adhesion of the sliding plate and sleeve increases due to the slow movement of the sleeve, there arises a problem that the rotation of the sleeve cannot follow the rotation of the drive roller.
Specifically, there arises a phenomenon that a toner image distorts since a slip arises between a surface of the glossy paper or OHT film, and a sleeve surface at the low-speed printing.
This phenomenon tends to arise in a state that the sliding resistance between the inside surface of the sleeve and a sleeve guide is large, and the frictional resistance between the surface of the OHT film and the sleeve surface is lowered. In an actual use, the severest state is a state that plenty of toner images are recorded on the OHT film.
In addition, not to say a defective image due to complete slipping, an unusual sound that is called a stick-slip (stick-slip phenomenon) may arise since the minute drive irregularity between the sleeve surface and a surface of the sliding plate causes the vibration between the inside surface of the sleeve and the sliding plate.
Such a stick-slip phenomenon remarkably arises in a state of low speed and high applied-pressure, which is a phenomenon quite contradictory to the gloss improving means for color image quality improvement.
As described above, in a fixing apparatus of a color image forming apparatus that fixes a full color image where plenty of toner is attached, it is necessary to apply the applied pressure, which is larger than that of a fixing apparatus for a conventional mono-color image, to the nip section so as to improve the gloss mode and the permeability of an OHT image. Hence, since the bearing stress between a rotating body and the supporting member in the nip section become larger, frictional resistance becomes large in particular. Therefore, the above-mentioned slip and stick-slip become very serious problems in the color image forming apparatus.